Supercontinuum generation in glass waveguides

Long-standing themes in the Division have been the development of novel supercontinuum sources, the study of the nonlinear dynamics of SC generation, and the development of single-shot spectral characterisation tools, for a variety of different systems using different glasses.

Chalcogenide glasses, which are transparent out to beyond 10 µm and have very high Kerr nonlinearities, are promising for extending supercontinuum spectra into mid-infrared. We have developed a double-nanospike structure by integrating nanotapers at both ends of a capillary waveguide filled with As2S3 glass (the nanotapers greatly enhance the in- and out-coupling efficiencies). Such a As2S3-silica double-nanospike waveguide was pumped by femtosecond pulses from a Cr:ZnS laser at 2.35 μm (repetition rate of 90 MHz) [Xie (2016)]. To match the waveguide GVD to the pump wavelength, two different samples, with core diameters 3.2 and 1.3 μm, were used in the experiments. At the pump wavelength, the 3.2 μm sample had weak anomalous dispersion, whereas the 1.3 μm sample had strong anomalous dispersion, with two zero dispersion points on opposite sides of the pump wavelength.

(a) Measured SC spectra generated by the As2S3-silica double-nanospike waveguide at different pump pulse energies for the 5-mm-long waveguide with 3.2 µm core diameter. (b) Simulated SC spectrum at the output face of the waveguides. (c) Simulated SC spectral evolution along the waveguide. (d)(e)(f), Results for 3-mm-long waveguide with 1.3 µm core diameter. The dark-red solid curves in (b) and (e) plot the calculated degree of coherence of the SC at the output face of the waveguides.

Silica-based photonic crystal fibre (PCF) has proven highly successful for supercontinuum (SC) generation, with smooth and flat spectral power densities. It suffers, however, from strong material absorption in the mid-IR (>2500 nm), and optical damage (solarisation) in the UV (<380 nm), which limits its performance and lifetime. SC generation in silica-PCF is therefore only possible between these wavelength limits. Although glasses such as chalcogenides, fluorides and heavy-metal oxides have been used to extend the spectrum out to longer wavelengths in the mid-IR, none have enhanced the UV performance. In 2015, we reported the first ZBLAN PCF with a high air-filling fraction, a small solid core, nanoscale features and a near-perfect structure. We used this PCF to generate an ultrabroadband, long-term stable, supercontinua spanning from 200 to 2500 nm [Jiang (2015)].

Recently we have continued to explore open issues such as the physics of UV SC light generation and dispersion tailoring in novel structures, in various ZBLAN PCFs. Due to the limited access to raw materials, we have focused on simplified structures, for example a six-core ZBLAN fibre with nanobores in each core has been used for SC generation [Jiang (2016)], as shown in Fig. 24. The cores had diameters of ~1.3 μm, with nanobore diameter ~330 nm. Spectral broadening was observed when a single core was pumped in the fundamental and first higher order core-modes with 200 fs-long pulses at a wavelength of 1042 nm. The results were verified by numerical simulations.

(a) and (b) Scanning electron micrographs of the cross-section of the ZBLAN-PCF. (c) Transmission spectrum of a ZBLAN bulk glass sample ~2 mm thick. (d) Supercontinuum spectrum generated by junction A for a launched pump energy of 830 pJ. (e) A magnified plot of the UV section of the SC spectrum, with a linear vertical scale. The inset photo shows the spectrum after being dispersed by a fused silica prism and cast onto a white screen.

Shot-to-shot spectral information at ~100 MHz repetition rate is possible using dispersive Fourier transformation (DFT). To access the spectral phase we have combined DFT with interferometry, which allows reconstruction of the shot-to-shot phase information of the pulse traveling in a cavity at 80 MHz repetition rate. The concept is based on spectral reconstruction of the output of a Mach-Zehnder interferometer (MZI) in which an all-normal dispersion fibre (BF) is inserted in one arm (reference arm), the passive ring cavity being in the second arm. The output of the MZI is monitored shot-to-shot by DFT detection, consisting of 2 km of dispersive fibre and a 12 GHz photodiode connected to a fast oscilloscope. In the reference arm the input spectrum is broadened by self-phase modulation so that the overall spectrum is linearly chirped, which eases reconstruction of the phase at the output of the MZI. Operating the passive ring cavity in a period-2 state, where the energy of a circulating pulse reproduces every other round trip, we could see a rearrangement of both the spectrum and the phase. This technique can also be applied to much more complex dynamics, for example, we were able to reconstruct the phase of consecutive pulses for a period-14 state [Hammer (2016)].

(a) Experimental set-up. A solid-core PCF is incorporated into a ring cavity terminated by two beam splitters (IC and OC). Temporal walk-off between the intracavity pulse and the pump pulse is adjusted by varying the delay in the ring cavity. Input pulses are split between the ring-cavity in one arm of the Mach-Zehnder interferometer and the second arm where an all-normal dispersion fibre (BF) is inserted. The shot-to-shot energy of the pulse traveling in the cavity is recorded with the fast photodiode PD1. The DFT is placed at the end of the MZI to record phase information.